In the forthcoming evolution of the mobile cellular standards, new transmission techniques like OFDM (Orthogonal Frequency-Division Multiplexing) will be used. Furthermore, in order to have a smooth migration from existing cellular systems to new high capacity high data rate systems in existing radio spectrum, a new system has to be able to operate in a flexible bandwidth. A proposal for such a new flexible cellular system is denoted the 3rd Generation Long Term Evolution (3G LTE) and can be seen as an evolution of the 3G WCDMA (Wideband Code Division Multiple Access) standard. This system will use OFDM as multiple access technique (called OFDMA) in the downlink and single carrier frequency division multiplexing (SC-FDMA) in the uplink. In both downlink and uplink the LTE will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s in the downlink and 50 Mb/s in the uplink will be supported for the largest bandwidth. However, not only high rate services are expected to use 3G LTE, but also low rate services like voice. Since 3G LTE is designed for the Internet protocol suite TCP/IP, Voice over Internet Protocol (VoIP) will be the service carrying speech.
Another important aspect of LTE is the mobility function as described in document 3GPP TS 36.300, “Overall description; stage 2”. This means that cell search as well as efficient neighbour cell measurements is of major importance in order for a user equipment (UE) to be able to stay connected to a suitable serving cell in the cellular communication system.
In release 8 of the 3GPP-specifications the base station can be equipped with 1, 2 or 4 transmit antennas. UE support for receiving signals from 1, 2 or 4 is mandatory but it is up to the base station to deploy 1, 2 or 4 transmit antennas. A deployment scenario with equal number of transmit antenna ports 10 at the base station sites 11 in a coverage area is shown in FIG. 1a and a deployment scenario with unequal number of transmit antenna ports at the base station sites 11 in another coverage area is shown in FIG. 1b. Use of more transmit antennas at the base station improves system performance by providing spatial diversity especially in radio environments of certain characteristics e.g. stationary situation (e.g. when the UE 12 is slow moving or static) or in radio environments with very low delay spread. However, an increase in the number of transmit antennas at the base station may also increase cost, complexity, signalling overheads etc. Therefore due to the factors such as desired system capacity, channel environment, cost consideration etc different number of transmit antennas are likely to be used in typical network setup.
An unequal number of antennas used at different base station sites also impacts the neighbour cell measurements. In LTE, information related to the exact number of antennas used at neighbour cells is not signalled to the UE because the signaling of an explicit neighbour cell list, which would contain neighbour cell specific information such as number of antennas, transmission bandwidth etc, is not mandatory in LTE. Thus in typical LTE deployment the network will not provide the UE with a neighbour cell list including the number of antennas used at the neighbour cells. In the absence of explicit neighbour cell antenna information, the UE will have to either read system information of the neighbour cell or blindly detect the presence of additional antennas. The blind detection requires that the UE performs correlation over expected reference signals (or pilot symbols) belonging to the additional antenna(s). As previously mentioned, one main advantage of use of multiple transmit antennas is to achieve spatial diversity.
In stationary conditions (i.e. when UE is stationary or moving very slowly) use of one antenna for neighbour cell measurements would not provide any time diversity. Thus, signals undergoing a fading dip will stay consistently or at least for a considerable period of time at lower received level. There is high risk that a UE camps on or perform handover to a weaker cell resulting in loss of received SNR (signal-to-noise ratio). In such circumstances it is important that the UE uses more than one antenna for neighbour cell measurements. It is also important to note that LTE is envisaged to provide broadband coverage to static or quasi-static users. Hence in typical LTE network there will be considerable number of users operating at negligible or very low speed but at the same time using higher data rate services.
In LTE, Reference Signal Received Power (RSRP) and/or reference signal received quality (RSRQ) are used for handover measurements, i.e. the UE needs to measure RSRP and RSRQ on the serving cell as well as on detected (by cell search) neighbouring cells. RSRP is defined as the average signal power of the transmitted Reference Symbols or Signals (RS) (transmitted by the eNode B). RSRQ is the ratio of RSRP to RSSI, where RSSI is the total received power from serving cell, non serving cells as well as from all other noise sources. RSRQ is a function of RSRP so for brevity we will focus on RSRP in the proceeding sections.
The RS's are transmitted from the radio base station i.e. Node B from each of possibly 1, 2 or 4 transmit antennas, on certain Resource Elements (RE) in the time-frequency grid, i.e. in some sub-carriers (every 6th) in OFDM symbol 0 and 3/4 (long/short CP) in every slot (consisting of 6/7 OFDM symbols). Furthermore, the RS in symbol 3/4 is offset by 3 sub-carriers relative to the RS in the first OFDM symbol. FIG. 2 shows the REs used for RS for transmit antenna 1 (denoted R) and a potential transmit antenna 2 (denoted S). Ideally, the RSRP should be based on RS from transmit (TX) antenna 1 and 2 (if 2 or more TX antennas are used). However, the number of TX antennas used for a detected neighbouring cell (i.e. a cell detected in the cell search procedure but the UE currently not connected to) is typically not known in advance for the UE and need to be blindly detected. For low SIRs, (i.e. the typical case for neighbouring cells, having signal power lower than the serving cell) there is a significant risk of erroneous detection of the number of TX antennas, making RSRP measurements based on a potential second TX antenna highly unreliable having a potentially large bias. The straightforward solution to this problem is to do a more advanced TX antenna detection algorithm (to be sure to detect the number of eNode B antennas) which typically means long time, power and/or hardware consuming FFT (fast Fourier transform) processing of neighbouring cell data which directly impact the signal power estimate processing complexity and therefore not desirable.
Once the UE has performed measurements on neighbouring cells that fulfill certain quality, the UE provides the measured result to the network (usually known as the measurement report). The result includes cell identities of the cells fulfilling conditions set by the network as well as RSRP result for each of the cell.
For determination of the RSRP estimate it is typically assumed that the channel for sub carriers is constant over a certain number of sub-carriers (i.e. in frequency) and OFDM symbols (i.e. in time) (see FIG. 2) and does coherent average over such a “constant” area or time-frequency grid to get a channel estimate Hi. In FIG. 2, Yi, Ri and Ei are the received reference signal at the UE, transmitted signal (i.e. reference signal) by the eNode B and noise signal respectively. Then, the absolute square |Hi|2 is taken to obtain a signal power estimate over such a block (i.e. “constant” area or time-frequency grid) and a non-coherent average over such signal power estimate blocks over the entire measurement bandwidth (typically 1.4 MHz, or 6 resource blocks) is performed to determine the total RSRP estimate. However, in case of delay spread and/or Doppler shift, the channel will not be perfectly constant over such a coherent block, leading to a bias in RSRP estimates. This problem could be addressed by using more advanced channel and signal power estimate methods (for instance based on advanced filtering techniques), which, however, also implies the drawback that these advanced methods are complex and heavy computations need to be done on each detected neighbouring cell. This in turn increases UE complexity and also leads to increase in UE power consumption.